The present invention relates to composite multilayer constructions, and more particularly, to multilayer pressure-sensitive adhesive constructions for labels comprising two or more adhesive layers.
The pressure-sensitive adhesive (PSA) label industry is unique and different from the tape industry in terms of materials, manufacturing conditions, product and product applications. PSA label products generally comprise a release liner coated with a low surface energy material such as silicone, a PSA layer disposed onto the liner, and a facestock adhesively bonded to the PSA layer. Such PSA label constructions may be formed by first coating or laminating the PSA to the release surface of the liner and then adhering the facestock to the PSA-coated liner; or alternatively, by coating or laminating the PSA to the facestock and then contacting the PSA-coated facestock with the release coated liner. The facestock is characteristically made from a web or sheet of paper or plastic, which can be printed with information or other indicia either before or after it is laminated to the PSA layer.
In a typical process for xe2x80x9cconvertingxe2x80x9d the label construction, the facestock is printed, the facestock and adhesive die-cut down to the liner surface to outline the label shape, and the waste material between the labels (matrix) is removed by stripping. The PSA label facestock and the adhesive is then adhered to a substrate surface by separating the label from the liner and causing the PSA layer of the label to come into contact with the substrate surface. In the most popular labeling process, the label is separated from the release liner by bending the liner back over a peel plate, whereupon the label is sufficiently stiff to cause the label to continue on a straight path toward the desired substrate surface.
As used in this patent application, xe2x80x9cseparationxe2x80x9d refers to removal of the label from the liner; xe2x80x9capplicationxe2x80x9d refers to adhesion of the label to the substrate surface; and xe2x80x9cdispensingxe2x80x9d or xe2x80x9cdispensabilityxe2x80x9d refers to the combined steps of separation and application. xe2x80x9cPeel plate dispensingxe2x80x9d denotes the use of a peel plate in the separation of the liner from the label.
The PSA label performance is primarily dominated by the ability of the adhesive to wet the application substrate to establish adequate surface contact and bonding strength between the adhesive and the substrate when they are brought into contact. These desirable results are accomplished by the visco-elastic properties and the surface chemistry of the adhesive. The adhesive may be chosen to provide a strong interfacial interaction with a wide variety of substrates and surfaces (e.g., low energy, high energy, polar or non-polar surfaces). It is also desirable that the adhesive exhibit a viscous nature at low deformation frequencies (e.g., 10xe2x88x921 to 101 Hz) during label dispensing applications so that, under pressure, the adhesive is able to flow or wet the substrate in order to have adequate adhesive-substrate interfacial contact to establish bonding. At higher deformation frequencies (e.g., 103 to 106 Hz) during label conversion, however, the adhesive must exhibit an elastic nature so that the adhesive can be cut evenly by the converting die.
Fillers are particulate additives which can be added to adhesives to increase the bulk of the adhesive and/or to improve properties. Fillers are commonly used in adhesive formulations for the following major reasons: (1) for cost reduction such as by adding calcium carbonate, clay or talc; (2) for coloring the adhesives such as by the addition of titanium dioxide, colored toner and security taggants; and (3) for performance enhancement such as by increasing stiffness and tensile strength, reducing cold flow, reducing edge flow, improved cutting, etc. Filler particles may be either inert-non-reinforcing fillers or active-reinforcing fillers. Inert-non-reinforcing fillers or extenders are fillers that do not strongly interact with the adhesive polymer resin. Examples include untreated clay, talc, calcium carbonate and titanium dioxide. They are usually included in the adhesive to increase adhesive bulk, reduce the cost, or change the appearance or chemical resistance of the adhesive. Such fillers also improve the handleability of the polymer resin mixture. The active-reinforcing fillers are those which interact with the adhesive polymer resin by promoting, for example, additional cross-linking or filler-polymer interface interaction. Examples of such fillers include surface-treated clay, zinc oxide, fine particle size silica and carbon black. The reinforcement is usually defined in terms of an increase in mechanical properties such as tensile strength or modulus, and occasionally, adhesion properties.
In tape applications where the adhesive layer is thick and may be in the range of from a few hundred microns to a few thousand microns in thickness, fillers are commonly used to obtain benefits such as lower cost and higher stiffness.
In the thin coatings utilized in label applications (generally under 100 microns in thickness), however, large amounts of fillers will cause significant loss of tack. Therefore, the amount of filler used in label applications is limited. Adhesive performance and anchorage to the facestock may suffer significantly by the inclusion of only moderate amounts of added fillers. For example, the addition of fillers to thin PSA films generally results in a decided deadening effect.
The present invention relates to improved performance multilayer PSA constructions useful for making labels. In one embodiment, the invention relates to a multilayer PSA construction comprising:
(A) a multilayer adhesive laminate having an overall thickness of less than about 100 microns and comprising
(i) at least one composite PSA layer comprising a continuous phase of a pressure-sensitive adhesive and a discontinuous phase of non-adhesive filler particles, filler particle aggregate, or a mixture thereof, and
(ii) at least one second PSA layer which is in contact with the composite layer and contains no filler or less filler than the composite layer, and
(B) a facestock which is in contact with and adhered to either the composite layer or the second PSA layer of the multilayer adhesive laminate. Preferably, the overall thickness of the multilayer adhesive laminate is from about 5 to about 75 microns, and the composite pressure-sensitive adhesive layer contains from about 5 to about 500 phr of filler. In other embodiments, the multilayer PSA label constructions also comprise a release liner in contact with the adhesive layer which is not in contact with the facestock. The proper distribution of the filler between the two or more PSA layers, and, more particularly, the inclusion of the filler in only one of the two PSA layers improves the performance of the adhesive such as by improved adhesion to the facestock or a substrate, adhesive drying rate, and label converting capabilities (e.g., improved cutting).
Die-cut labels also are described which are prepared by die-cutting the die-cuttable and matrix-strippable adhesive composite constructions of the invention, particularly those constructions described herein which comprise a multilayer PSA label construction in combination with a substrate having a release surface such as a release liner.